1. Field of the Invention
The present invention relates to a method of producing alkali metal azides useful as a raw material for the production of tetrazoles which can be used as an intermediate material for medicines, as a raw material for the production of organic azides useful for light sensitive materials for photo masks employed in the field of electronics industries, or as a major raw material for inflaters used in air bags for cars.
2. Description of the Prior Art
It has long since been known to produce alkali metal azides by reacting hydrazine with alkyl esters of nitrous acid in solvents in the presence of alkali metal hydroxides.
For example, U.S. Pat. No. 1,628,380 describes as prior art before the filing day thereof (a) a method in which the aforementioned reaction is conducted in non-anhydrous reaction mediums, and (b) the aforementioned reaction is performed in practically anhydrous reaction mediums.
According to the prior art method, the reaction of the former method (a) is considered to proceed according to reaction scheme-1 below: EQU N.sub.2 H.sub.4 .multidot.H.sub.2 O+RONO+NaOH.fwdarw.NaN.sub.3 +ROH+3H.sub.2 O (1)
wherein R represents an alkyl group.
In the case of the above reaction (1), a large amount of sodium azide together with excessive raw materials and one or more by-products is dissolved in the water-bearing or hydrous reaction medium, and hence a mineral acid such as sulfuric acid is added to the reaction solution to generate hydrogen azide according to reaction scheme-2 below: EQU 2NaN.sub.3 +H.sub.2 SO.sub.4 .fwdarw.2HN.sub.3 +Na.sub.2 SO.sub.4( 2)
and the hydrogen azide thus generated is absorbed with sodium hydroxide to produce sodium azide according to reaction scheme-3 below: EQU HN.sub.3 +NaOH.fwdarw.NaN.sub.3 +H.sub.2 O (3)
However, according to the aforementioned U.S. patent, while the method (a) described above is advantageous in that crude raw material which are less expensive can be used, it requires many steps including a step of distilling hydrogen azide, which is not only dangerous but also involving much loss.
On the other hand, the method (b) described above is considered to proceed according to reaction scheme-4 below: EQU N.sub.2 H.sub.4 +RONO+RONA.fwdarw.NaN.sub.3 +2ROH+H.sub.2 O(4)
wherein R represents an alkyl group.
In this reaction, practically anhydrous raw materials such as alcohols, high concentration hydrazine and sodium ethylate must be used. Therefore, the method is considered dangerous and expensive because it inevitably involves the use of metal sodium in the preparation of sodium ethylate although it is advantageous in that the objective compound, sodium azide, can be isolated by simple filtering because of insolubility of sodium azide in the reaction medium.
The production method described in the U.S. Pat. No. 1,628,380 was intended to combine advantageous features of the conventional methods (a) and (b) described above, and hence is characterized by reacting a hydrazine hydrate, an alkyl nitrite and a sodium alcoholate solution together.
According to the example of U.S. Pat. No. 1,628,380, 1 kg of aqueous hydrazine solution containing 500 g (15.6 moles) of hydrazine, 7.49 kg of a solution of 7.5% by weight sodium hydroxide in anhydrous ethanol, and 1.76 kg of ethyl nitrite were reacted to obtain 0.8 kg of sodium azide of a purity of 90% by weight or more. The yield of the product of this reaction amounts to only 79% based on the weight of hydrazine used as raw material, even if the purity of sodium azide thus obtained was assumed to be 100% by weight.
According to the description on page 2, lines 25 to 32 and claim 4 of the U.S. patent, the concentration of hydrazine in the aqueous hydrazine solution used should be high enough so that the proportion of hydrazine to water can be maintained at least 1 unit of hydrazine to 3 units of water including the water generated in the reaction. According to the aforementioned example of the prior art a relatively large amount of the ethanol solution as much as 7.49 kg as compared to 1 kg of the aqueous hydrazine solution is used, and hence the solvent used in the reaction system described in U.S. Pat. No. 1,628,380 is an alcoholic solvent composed of a smaller proportion of water and a larger proportion of an alcohol.
As stated above, the proposal described in the aforementioned U.S. patent uses an alcoholic solvent containing a very small amount of water as the reaction solvent, and basically relates to improvement of the latter method (b) to the prior art relative to the invention described in the U.S. patent, and it is recommended to perform the reaction in reaction systems in which contamination of water has been minimized as far as possible practically. Therefore, when the production method is used industrial production, the method must involve additional steps of recovering the solvent used, purifying it and dehydrating it. Further, yield is not always sufficient. As a result, the production method is disadvantageous practically.
As stated above, the conventional industrial production method of producing alkali metal azides from hydrazine have the following defects:
(1) The use of anhydrous alcohols as the solvent involves cost for recovering the solvents;
(2) Non-recovered solvents (i.e., loss of solvents) causes increase in the production cost of the product;
(3) Special installation is needed for handling combustible solvents;
(4) Yield of the product for hydrazine is not always sufficient; and the like.